The concepts of harmonic sampling, quadrature mixing and sinusoidal transforms are well known methods for conditioning signals for processing in computer devices. The DSSS DSP receiver uses these concepts with the innovative teachings of this document to receive, despread, detect and demodulate a direct sequence signal using DSP microprocessor type hardware structures.
U.S. Pat. No. 5,067,136 issued Nov. 19, 1991 teaches a "Wireless Alarm System", contemplating a spread spectrum receiver which:
Converts the received signal to lower frequencies utilizing the RF section; PA1 Includes a chip code generator with means of chip code phase shifting for achieving correlation lock; PA1 Teaches means to measure both signal strength and quieting to detect correlation lock over the dynamic range of the system; PA1 Utilizes an adaptive data demodulator tolerant to DC; i.e. long strings of 1's or 0's; and PA1 Implementing microprocessor algorithms to perform the above.
See particularly FIGS. 3A and 4 of '136, teaching respectively, a block diagram of a spread spectrum receiver, and a flow chart of the code locking algorithm. The present, applied for invention builds upon some of the basic concepts embodied in this system, nonetheless teaching improvements which are believed patentable on their own.
In the system of the present, applied for invention, the signal is received and converted to an intermediate frequency (IF) for sampling by an analog to digital converter (ADC). The methods for conditioning the signal for use by the converter are well known in the art and not part of this teaching. The specific implementation described herein does make one interesting if not novel use of the converter in that the ADC is used as the second downconversion mixer in the radio.
It is therefore an object of the present invention to provide a DSSS DSP implementation technique requiring relatively nominal computational hardware and software requirements.
It is another object of the present invention to provide a unique energy detection algorithm for discerning spread spectrum while in the search mode, sampling in a matter which increases strength, reduces noise, and changes the relative phase of the received chipping code with respect to the received signal.
It is still another object of the present invention to provide a unique PRC code filtration technique, resulting in minimal despreading loss, while providing interference rejection of CW signals greater than 1.5 MHz from the center IF frequency.